Control of the spacings and mechanical forces within the construction of a flat panel display (i.e., liquid crystal displays, electroluminescent displays, vacuum fluorescent displays, field emission displays, and plasma displays) is often critical to the performance of the corresponding device and depends upon the incorporation of physical spacers into the corresponding display. For example, in liquid crystal displays (LCDs), the polarization of the light exiting the display is controlled in part by the optical path length through the liquid crystal layer. In current display technology, the thickness of the liquid crystal layer is determined by spacers, which may be in the form of particles (i.e., spherical beads or fibers), columnar structures (i.e., posts or pillars), microribs, etc. Spacers have become increasingly important with the desire for light-weight large format displays. To achieve lighter weight display panels, transparent polymeric substrates are typically used since they are much lighter than glass. However, polymeric substrates are more flexible; thus, requiring a denser population of spacers to maintain a uniform thickness throughout the display panel. The most common and inexpensive method for controlling the thickness of the liquid crystal layer is to deposit a random arrangement of particles having a narrow size distribution over the entire surface of the substrate or alignment layer. This process has an obvious disadvantage in that there is no control over the placement of the particles resulting in a high percentage of the particles appearing in the display windows, thus decreasing the amount of light that may pass through the display. In many applications, the particles are not anchored to the substrate and may shift or migrate causing artifacts to appear in those areas in the display cell. The spray application presents an additional issue in the manufacturing process. The display is assembled in Class 10 to 100 cleanrooms to meet the optical quality requirements for the liquid crystal displays. Spraying particles onto a surface results in many of the particles becoming airborne, thus making it difficult to maintain Class 10 to 100 standards. The thinner the layer desired the smaller the particle required which leads to increased handling and application difficulties.
One attempt to overcome the deficiencies in liquid crystal displays as described above is disclosed in U.S. Pat. No. 4,720,173 and Japanese Patent applications, JP 7325298, JP 5203967, and JP 2223922 where a photoresist material is bonded to the substrate, imaged and developed to generate spacer entities. This method allows one to more precisely place the spacer on the substrate; however, the requirement of a developing step adds an additional step to the process. Liquid development also produces spent developer solutions which must be disposed of. Many of the developers contain solvents or have a high pH, thus requiring special handling for safety and/or special disposal to meet federal and state environmental regulations. It is also more difficult to maintain a uniform thickness of the spacers when a photoresist is used. For example, the developer may etch away more of the surface in one area than in another.
An alternative approach for controlling spacing in liquid crystal displays is described in U.S. Pat. No. 5,268,782; where, a microstructured substrate is used as both a substrate and a spacer integrated into one element. To minimize interferences in the window areas, the microstructured surface typically comprises a series of parallel ridges (microribs). Even though the percentage of spacers within the optical window is minimized, a stripping effect is visible in the display. Additionally, the deposition of the high viscosity liquid crystals is more arduous when microribs are used for spacers. For instance, it is harder to apply the high viscosity liquid crystals without entrapping air which creates an optical defect in the layer.
Clearly there is a need for a method and materials for accurate placement of structurally supporting spacers which are cost effective, reliable, and eliminate interference with the optical integrity of the display panel.